The Wnt gene family encodes a large class of secreted proteins related to the Int1/Wnt1 proto-oncogene and Drosophila wingless (“Wg”), a Drosophila Wnt1 homologue (Cadigan et al. (1997) Genes & Development 11:3286-3305). Wnts are expressed in a variety of tissues and organs and play a major role in many developmental processes, including segmentation in Drosophila; endoderm development in C. elegans; and establishment of limb polarity, neural crest differentiation, kidney morphogenesis, sex determination, and brain development in mammals (Parr, et al. (1994) Curr. Opinion Genetics & Devel. 4:523-528). The Wnt pathway is a master regulator in animal development, both during embryogenesis and in the mature organism (Eastman, et al. (1999) Curr Opin Cell Biol 11: 233-240; Peifer, et al. (2000) Science 287: 1606-1609).
Wnt signals are transduced by the Frizzled (“Fz”) family of seven transmembrane domain receptors (Bhanot et al. (1996) Nature 382:225-230). Wnt ligands bind to Fzd, and in so doing, activate the cytoplasmic protein Dishevelled (Dvl-1, 2 and 3 in humans and mice) (Boutros, et al. (1999) Mech Dev 83: 27-37) and phosphorylate LRP5/6. A signal is thereby generated which prevents the phosphorylation and degradation of Armadillo/β(beta)-catenin, in turn leading to the stabilization of β-catenin (Perrimon (1994) Cell 76:781-784). This stabilization is occasioned by Dvl's association with axin (Zeng et al. (1997) Cell 90:181-192), a scaffolding protein that brings various proteins together, including GSK3, APC, CK1, and β-catenin, to form the β-catenin destruction complex.
The Wingless-type (Wnt) Frizzled protein receptor pathway involves important regulatory genes that carry polymorphisms associated with primary carcinomas. In the course of downstream signaling, cytosolic β-catenin accumulates, translocates into the nucleus, and then enhances gene expression by complexing with other transcription factors Uthoff et al., Mol Carcinog, 31:56-62 (2001). In the absence of Wnt signals, free cytosolic β-catenin is incorporated into a complex consisting of Axin, the adenomatous polyposis coli (APC) gene product, and glycogen synthase kinase (GSK)-3β. Conjunctional phosphorylation of Axin, APC, and β-catenin by GSK-3β designates β-catenin for the ubiquitin pathway and degradation by proteasomes Uthoff et al., Mol Carcinog, 31:56-62 (2001); Matsuzawa et al., Mol Cell, 7:915-926 (2001).
Disheveled (Dvl) is a positive mediator of Wnt signalling positioned downstream of the frizzled receptors and upstream of βcatenin. GSK-3 phosphorylates several proteins in the Wnt pathway and is instrumental in the downstream regulation of βcatenin. Mutations in the gene APC are an initiating event for both sporadic and hereditary colorectal tumorigenesis. APC mutants are relevant in tumorigenesis, since the aberrant protein is an integral part of the Wnt-signaling cascade. The protein product contains several functional domains acting as binding and degradation sites for βcatenin. Mutations that occur in the amino-terminal segment of βcatenin are usually involved in phosphorylation-dependent, ubiquitin-mediated degradation and, thus, stabilize β catenin. When stabilized cytoplasmic-catenin accumulates, it translocates to the nucleus interacting with the Tcf/Lef high-mobility group of transcription factors that modulate expression of oncogenes such as c-myc.
It is known that Wnt/β-catenin signaling promotes cell survival in various cell types Orford et al., J Cell Biol, 146:855-868 (1999); Cox et al., Genetics, 155:1725-1740 (2000); Reya et al., Immunity, 13:15-24 (2000); Satoh et al., Nat Genet, 24:245-250 (2000); Shin et al., Journal of Biological Chemistry, 274:2780-2785 (1999); Chen et al., J Cell Biol, 152:87-96 (2001); Ioannidis et al., Nat Immunol, 2:691-697 (2001). Wnt signaling pathway is also thought to be associated with tumor development and/or progression (Polakis et al., Genes Dev, 14:1837-1851 (2000); Cox et al., Genetics, 155:1725-1740 (2000); Bienz et al., Cell, 103:311-320 (2000); You et al., J Cell Biol, 157:429-440 (2002)). Aberrant activation of the Wnt signaling pathway is associated with a variety of human cancers, correlating with the over-expression or amplification of c-Myc (Polakis et al., Genes Dev, 14:1837-1851 (2000); Bienz et al., Cell, 103:311-320 (2000); Brown et al., Breast Cancer Res, 3:351-355 (2001); He et al., Science, 281:1509-1512 (1998); Miller et al., Oncogene, 18:7860-7872 (1999). In addition, c-Myc was identified as one of the transcriptional targets of the β-catenin/Tcf in colorectal cancer cells (He et al., Science, 281:1509-1512 (1998); de La Coste et al., Proc Natl Acad Sci USA, 95:8847-8851 (1998); Miller et al., Oncogene, 18:7860-7872 (1999); You et al., J Cell Biol, 157:429-440 (2002)).
Thus, a need exists for agents and methods that modulate the Wnt signaling pathway, thereby treating, diagnosing, preventing, and/or ameliorating Wnt signaling-related disorders.